A campaign group calling for fairer fuel prices has slammed a sharp increase on "greedy opportunistic oil speculators".

It comes after motoring group the RAC revealed the average cost of a litre of petrol rose by 6p in May - the highest price hike since it began tracking prices 18 years ago.

It says the average cost of a litre rose to 129.4p with diesel prices rising a similar amount to 132.3p.

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The news enraged Howard Cox, founder of the Cranbrook-based FairFuelUK Campaign.

He said: "Current pump prices are 1-2p per litre more than they need be, despite escalating oil costs and the weaker pound.

"Greedy opportunistic oil speculators along with OPEC and Russia's oil production levels posturing, are using geo-political spats between the USA and Iran to deliberately over inflate the cost of filling up.

"Businesses in the fuel supply chain are yet again exploiting the already highest taxed motorists in the world by not reducing pump prices when oil prices fall. They rise like a rocket and fall like a feather.

"As a consequence, inflation, GDP and consumer spending is being impacted badly by these greedy companies.

"Reports from FairFuelUK supporters are showing that in some cases, garages have put up prices three to four times in May, when fuel forecourt owners have only paid for one to two deliveries of bulk fuel in the same month.

"How can that be fair, or even honest?

"Along with MPs across all parties, we call on the government to set up an independent price monitoring body to make pricing changes at the pumps, transparent."

The RAC says the rises takes the cost of filling up a 55-litre family car to £71.18 - an increase of £3.29 in just one month.

The RAC Fuel Watch data also shows the average prices of both petrol and diesel have gone up every single day since April 22, adding 8p a litre in the process – the longest sustained price increase since March 2015.

In May, it says, the big four supermarkets raised petrol by 5.49p a litre and diesel by 5.88p while on the motorway, service stations added 6.37p to unleaded, taking it to 144.75p a litre, and 6.69p to diesel making it an eye-watering 147.80p a litre – 15p a litre above average UK prices for both fuels.

RAC fuel spokesman Simon Williams said: “May was a hellish month for motorists. Sadly, they have been besieged by pump price rises for three months with nearly 9p a litre being added to petrol since the beginning of March.

“The rising oil price together with a weaker pound is a punitive combination for anyone that drives regularly. For many people there is little alternative to the car for the majority of journeys they have to make so it is therefore very difficult to avoid feeling the pinch of rising pump prices.

“In the last week of May the oil price cooled a little to $76 a barrel which is slightly better news for motorists as the RAC’s two-week forecast is currently showing that average prices may even reduce by a penny or so.

"While this isn’t much, and could easily change in response to oil trading this week, it is at least a sign that the constant rise in forecourt prices may have stopped for the time being.

“Looking at the bigger picture, there is talk that OPEC – the Organization of the Petroleum Exporting Countries – may agree at its meeting on 22 June in Vienna to change its tack of restricting oil supply.

"The group, together with Russia, have been limiting production with a view to removing the long-term oil glut.

"This strategy has been successful and, as intended, caused the barrel price to rise. If a decision is taken to increase supply it may provide some much-needed relief for motorists at the pumps in the UK.”